1. Field of the invention:
This invention applies to shaping parts in the manufacture of flat tension mask color cathode ray tubes. More specifically, the invention describes a means for forming a smooth, uniform and burr-free surface to which a tensed foiled shadow mask will be permanently affixed.
In particular the invention relates to a portion of the process steps employed in the manufacture of a front assembly of a flat tension mask color cathode ray tube. A front assembly includes a flat glass panel, planar on one side defined as the inner surface of the panel, a support structure affixed to the inner surface of the panel and a tensed foil shadow mask permanently attached to the support structure with proper registration relative to the color emitting phosphors, applied to the inner surface within the boundaries imposed by the support structure perimeter. Such a front assembly is described in U.S. Pat. No. 4,686,416.
2. Definitions
Certain definitions of terms and phrases used in this disclosure add to the clarity of the description of the invention.
Tube: The term "tube" in this disclosure means a flat tension mask cathode ray tube such as that used as a color television receiver screen or as a color computer monitor screen.
Panel: The front portion of the tube which functions as the viewing screen is a relatively thick flat glass plate.
Mask: An essential part of a tube is a thin metal shadow mask placed close to and behind the inner surface of the panel. In the preferred embodiment of this disclosure, the shadow mask is described as being made of steel and as being permanently affixed to a support structure by means of laser welding.
Support Structure and Rail: For proper functioning of the cathode ray tube, the mask is permanently installed, with proper orientation, in a plane at a specified distance from the inner surface of the panel. The support structure may be part of the panel or made of separate elements attached to the panel. In the illustrated preferred embodiment, the mask is permanently affixed to a support structure consisting of a continuous metal rectangular frame bonded to the panel by a process called fritting. In this document, the metal rectangular structure is called the "support structure". Any one side of the support structure is called a "rail".
Land and Mask Receiving Surface: In preparation for welding, the top surface of the support structure is ground flat. The ground surface of the support structure to which the mask is welded is called the "land" or the "mask receiving surface".
Fixture: The entire tooling system used to hold the panel during the grinding process as described in the preferred embodiment is called the "fixture".
3. Reference to Prior Art:
There is no known prior art related to forming a flat planar surface on the support structure for a flat tension mask cathode ray tube. Grinding of flat planar surfaces is well established in terms of available machinery and grinding media, but this application introduces requirements not encountered outside the field of manufacturing flat tension mask color cathode ray tubes.
4. Problems in the Manufacture of Flat Tension Mask Cathode Ray Tubes:
The materials of the parts to be held and ground prior to final assembly do not lend themselves to being clamped in position by common methods. The degree of precision and processes required for positioning the mask relative to the inner surface demand the unique methods described herein.
What follows is a number of the problems addressed and solved by the invention:
A. Checking and Cracking: If the glass in the panel is scratched during manual handling while being clamped in the grinding position, such surface blemishes induce stress concentrations at the scratches. This introduces the possibility of breakdown in the structure of the glass when the panel is subsequently subjected to high temperatures during the sealing of the tube. Such damage may render the tube inoperable or a total loss. To reduce the possibility of scratching the panel, holding clamps of the fixture must be applied directly to the panel without any rotational or sliding motions while the clamps are in contact with the panel.
B. Metal Softness: The support structure is preferably made of a 28% chromium-iron alloy commonly known of as "Carpenter Glass Sealing 27". It is difficult to grind such a material and produce a surface with edges free of burrs. Burrs tear the thin material of which the mask is made during the mask application and attachment process. Also, burrs may break free from the support structure of a finished tube and create charged particles which can cause electrical problems.
C. Accuracy: The mask is used as a photoexposure mask in the photolithographic processes for applying the black grille and three color emitting phosphors; therefore, use of the mask in said photolithographic processes dictates its precise repetitive placement and final permanent attachment in proper registration relative to the inner surface of the panel. The mask must be positioned a specified distance from all points of the inner surface during the photolithographic processes and again at final assembly. This specified distance is called the "Q" spacing of the tube and is related to electrical and mechanical geometry of the tube. For the purpose of illustration, the Q spacing of a typical 14" diagonal screen flat tension mask cathode ray tube is approximately 0.290". This dimension is used as the specified distance in the description of the preferred embodiment.
The panel is flat on the inner surface within an overall flatness specification of approximately 0.002" prior to the support structure fritting process. The 0.290" dimension must be maintained within a grinding tolerance on the order of plus or minus 0.002" at all points in the plane of the land.
D. Close Proximity: The 0.290" dimension demands that the grinding wheel operate within 0.290" of the inner surface, and that the inner surface be the reference surface. Therefore, the thickness of the holding clamps must be less than 0.290".
E. Grinding Forces: Grinding forces cannot exceed clamping forces.